pyside2: fixup build after qt5 update

in commit 202fff431f.  I blame upstream:
7f067fa8a5
(but I don't know these project, so can't know who's at fault)

Build error:
  qtdeclarative-5.15.7-dev/include/QtQuickWidgets/qaccessiblequickwidget.h:46:10:
    fatal error: 'private/qaccessiblequickview_p.h' file not found

pkgs/development/python-modules/pyside2/default.nix

@@ -23,6 +23,8 @@ stdenv.mkDerivation rec {
     "-DPYTHON_EXECUTABLE=${python.interpreter}"
   ];
 
+  NIX_CFLAGS_COMPILE = "-I${qt5.qtdeclarative.dev}/include/QtQuick/${qt5.qtdeclarative.version}/QtQuick";
+
   nativeBuildInputs = [ cmake ninja qt5.qmake python ];
   buildInputs = (with qt5; [
     qtbase qtxmlpatterns qtmultimedia qttools qtx11extras qtlocation qtscript